The present invention relates to tuned filters and, more particularly, to notch filters which are particularly suitable for use in removing one or more single or multiple frequency scrambling signals injected into a TV channel to eliminate the possibility of intelligible reception of video and/or audio information, or for removing a video carrier in a TV channel to prevent its reception.
A critical requirement of tuned notch filters, used in the above-described environments, is that the filters have a high degree of stability and reliability. In addition, the filters must also be economically constructed in such a manner to permit the filters to be quickly and precisely tuned to the proper frequency in order to ensure complete elimination of the signals to be removed. A further requirement of tuned notch filters is that the size of the filter must remain relatively small, preferably less than 0.825 inch diameter, to fit the application in a typical installation where one or more filters are mounted on a directional tap, on a strand, in a pedestal, or some other small enclosure.
Notch filters typically used in the environments described above must be capable of approximately 85 dB of attenuation at the center frequency of the notch with a very sharp skirt to minimize unwanted attenuation of adjacent channels. In order to tune such a filter, sophisticated and expensive tuning equipment is required to measure this large degree of attenuation across the filter. Tuning is further complicated by the fact that notch filters used for the purposes described above typically employ more than one filter section in cascade to achieve the high attenuation, wherein each of filter sections must be independently tuned. If sufficient isolation is not provided between the two filter sections, one filter section will affect the electrical performance of the other filter section due to magnetic coupling between the two sections.